TCF-S Temperature Compensating Focuser
TCF-S Temperature Compensating Focuser
TCF-S Temperature Compensating Focuser


TCF-S Temperature Compensating Focuser

Regular price
$ 875.00
Sale price
$ 875.00
Unit price

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Back in 1999 Optec's Gerald Persha designed and introduced the first all-digital motorized telescope focuser.  Featuring a closed-loop design with external hand control box and built-in temperature compensation, that very first TCF Temperature Compensating Focuser continues to operate every clear night at the Optec Observatory.  Utilizing a best-fit linear model to correct for temperature induced focal shifts, the TCF performs superbly without requiring a re-focus of the telescope for weeks at a time. 

As the first few focusers rolled out to customers, we quickly saw the need for PC computer control. By adding a serial UART chip and ASCII command set to the control circuit, the TCF-S was born.  Now focus control, along with serial "go-to" telescope mounts and imaging cameras could be controlled by a host of software packages.  

The remotely controlled observatory became a reality for many amateurs and small colleges using off-the-shelf products.  The revolutionary TCF-S was awarded a Sky & Telescope Hot Product award in 2000 for its contribution to this dramatic change in the way astronomy research and imaging was being accomplished. 

Since that time the TCF Focuser Series has grown to include larger focusers, focusers with integrated electronics, and new more durable controllers such as the FocusLynx which expands the serial protocols to include USB, Ethernet, and wireless WiFi capability.  Optec engineers continue the innovation with linear encoder versions for sub-micron positional accuracy.  Throughout all this growth, Optec has continually achieved the overarching goal of robust mechanical design with intelligent electronics for remote observatory operations.

#17670 - Original TCF-S Focuser with USB/Serial control

Mechanically, the TCF-S is a robust Crayford style motorized focuser with high repeatability, no play and nearly zero backlash of only 0.0013 inch. Optec's implementation is ideal for applications which require exact focus such as CCD imaging or film astrophotography. A geared stepper motor rotates the drive shaft with one step rotation of the motor equal to a 0.000086 inch movement of the drawtube. The unit adds 3.5 inches to the telescope's back-focus as measured from the front to the end of the drawtube when at the mid-focus position. The total travel of the drawtube is 0.6 inches or 7000 steps. A pair of push buttons control the direction of focus and the DRO (digital read-out) displays the current position. The TCF-S focuser can easily handle cameras and instrument packages weighing up to 10 pounds. The TCF-S can be used by itself or with a PC for automatic focus with some of the popular camera control software.

Unique to the TCF-S focuser, an electronic controller system monitors the telescope's tube temperature and compensates the focus accordingly. A small temperature probe is attached to the side of the telescope tube and monitors temperature with a resolution of 0.1° C. For a typical Schmidt-Cassegrain of 8 to 11 inches aperture and f/10 focal ratio, the back-focus will move approximately 0.010 inches for every 1° C change in temperature. It is not unusual during an observing session for the ambient temperature to change by as much as 10° C within the time span of a few hours. This change in focus due to temperature is a serious problem for most telescope designs and requires frequent re-focusing during long exposures. A typical RGB exposure sequence can last one hour or more making it imperative that the focus be checked and corrected after each filter change.

A simple setup procedure is used to find the temperature coefficients specific to the user's telescope system. The TCF-S system allows for two different coefficients (corresponding to two different f-ratio configurations) to be calculated and stored in the EEPROM memory. Once learned, either coefficient can be selected with a simple slide switch. A manual mode allows the user to set the focus manually at any time.

The digital nature of the TCF-S allows opportunities for truly intelligent focusing. With the TCF-S focuser, programmers can communicate directly with the focuser's controller (located in the hand control) via any serial port. The simple ASCII protocol is easily programmed and has been implemented in many popular imaging and camera control software packages. For example, CCDSoft Version 5 offers the @Focus routine which communicates with the TCF-S via a serial port to provide truly automatic focus control. MaxIm DL/CCD version 3 and above provides remote and automatic focus control with the TCF-S focuser.

At the end of an observing session, the TCF-S focuser remembers the last temperature and position. When the unit is turned back on for a new session, the TCF-S computes a new position using the current tube temperature and moves to that position. Assuming no changes to the optical configuration, the object will snap into sharp focus. It is possible to observe many days without the need to focus again.

The TCF-S comes with its own PC control program for Windows. The source code for the program is available in the TCF-S folder after the install program is run. Users are encouraged to write their operating program using the TCF-S.bas routines as examples. You can download the setup program for TCF-S_Control by going to our Software Download page. The navigation button is on the left.

 TCF-S/S3 Control Box

This control box powers the stepper motor, reads the temperature sensor, provides bi-directional serial communication with a PC and allow the user to remotely control and read the state of both TCF-S and TCF-S3 focusers. In order to attain proper connection, three cables must also be obtained: power, focuser control, and PC serial connection.

For normal operation, the MODE and LEARN slide switches are placed in the RUN and MANUAL position as shown in the diagram. To teach the focuser a new temperature coefficient, the LEARN switch is placed in the LEARN postion and the MODE switch is placed in AUTO A or AUTO B.

The position of the focuser in units of "steps" is shown on the DRO display. For the TCF-S, position ranges from 0 to 7000 steps which corresponds to a maximum travel of 0.60 inches. For the TCF-S3, the range of position is from 0 to 9999 steps which corresponds to a maximum travel of 1.00 inches.

Important Note! This control box is intended to be used to allow control of a focuser through a computer or from a desktop, not as a hand controller! For handheld operation of a focuser, purchasing the remote keypad is recommended.


  • Heavy-duty Crayford style 2" focuser 
    Compensates for focal shift due to temperature
  • Supports instruments up to 10 pounds
  • Easy to use control box
  • Use with or without a PC
  • 4-digit readout for position
  • Simple Learn procedure for determing TC
  • Shut-off procedure remembers last position
  • Compatible with major camera control software for autofocus
  • Free control/interface program
  • Click here for further technical details
  • Weight Capacity:    10 lbs. (4.5 kg) minimum
  • Overall Height:  3.3" (83.8mm) fully IN to 3.9" (99mm) fully OUT
  • Focuser Travel:  0.6" (15.2mm)
  • Number of Steps:  7,000
  • Step Resolution:  2.2 microns
  • Weight:  2 lb. 8 oz. (1.1 kg)
  • Clear Aperture:  2" (52.8mm)
  • Back-focus Usage:  3.3" (83.8mm) to 3.9" (99mm)

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